Fish turning black and dying

This is a discussion on Fish turning black and dying within the Tropical Fish Diseases forums, part of the Freshwater Fish and Aquariums category; -->
I have a 30 gallon BioCube tank that has been set up about 2 months.
Two of my Mollies and two of my Gouramis ...

I know you said parameters are in normal limits, but can you post the actual numbers? Also, what test kit was used? I have never had a fish turn black and die, but I just did a quick google search and found a few other forums where people suggested that turning black is a symptom of ammonia or nitrite burns.